1. Field of the Invention
The present invention relates to a vibration wave motor, and more particularly, to a vibration wave motor for driving a moving member by an elastic vibration member excited with a plurality of vibration modes.
2. Related Background Art
Up to now, the following vibration wave motors have been known. In the vibration wave motors, a plurality of vibrations obtained with a plurality of vibration modes are synthesized and a plate-shaped elastic vibration member is driven by the synthesized vibrations, thereby driving a moving member which is pressurized against the elastic vibration member into frictional contact therewith. As a representative example of the vibration wave motors, JP 6-311765 A discloses a vibration wave motor in which two bending vibration modes are synthesized.
In addition, JP 7-143771 A discloses a vibration wave motor in which a longitudinal vibration mode and a bending vibration mode are synthesized.
As an example of a pressurizing method of causing friction between the elastic vibration member and the moving member in such the vibration wave motor, JP 7-143770 A discloses a mechanical method using a spring or the like.
As disclosed in JP 59-185179 A, JP 4-088890 A, and JP 6-292374 A, there is also known a method using a magnetic force.
JP 11-285279 A and JP 2004-257844 A each propose a device for pressurizing the moving member by a magnetic force and guiding the moving member in a moving direction.
The largest feature of the plate-shaped vibration wave motor disclosed in each of, for example, JP 6-311765 A and JP 7-143771 A is that the vibration wave motor can be thinned.
In order to make full use of this feature, it is necessary to devise a pressurizing mechanism and a guide mechanism for the moving member.
For example, JP 7-143770 A discloses the pressurizing mechanism and the guide mechanism which are based on the mechanical method. However, it is difficult to thin the vibration wave motor.
Each of, for example, JP 59-185179 A, JP 4-088890 A, or JP 6-292374 A discloses the pressurizing mechanism using a magnetic force. The pressurizing mechanism is effective in thinning. However, the guide mechanism is not considered.
For example, JP 11-285279 A and JP 2004-257844 A each propose the device in which both the pressurizing mechanism and the guide mechanism of the vibration wave motor are constructed using a magnetic force.
That is, the device for performing pressurization by the magnetic force with a state in which the flow of magnetic flux of a permanent magnet is provided to be a closed magnetic circuit as proposed in JP 11-285279 A is desirable in view of the effective use of magnetic force. However, in order to form the closed magnetic circuit, it is necessary to oppose a square U-shaped linear guide and a square U-shaped yoke to each other, so that it is unsuitable to thin the device. The guide mechanism requires specific parts, which is disadvantageous in cost.
JP 2004-257844 A proposes a device for performing pressurization and guiding by the attractions of both strip-shaped permanent magnets with a state in which the permanent magnets are opposed to each other. Although a problem does not occur in the case of a rotational motor, when the attractions of the two combined permanent magnets are used in a linear motor, the following problems occur.
FIGS. 7A and 7B are side views showing two strip-shaped permanent magnets corresponding to parts of a conventional linear vibration wave motor.
For example, a permanent magnet (N-pole) 101 is fixed to a moving member and a permanent magnet (S-pole) 102 is fixed to an elastic vibration member. The permanent magnet (N-pole) 101 and the permanent magnet (S-pole) 102 are opposed to each other at a predetermined interval.
In general, a magnetic flux tries to flow so as to minimize a magnetic reluctance. When a relative positional relation between the permanent magnet (N-pole) 101 and the permanent magnet (S-pole) 102 is a positional relation shown in FIG. 7A, the magnetic reluctance becomes minimum. In contrast to this, when the permanent magnet (N-pole) 101 and the permanent magnet (S-pole) 102 are in a relative positional relation shown in FIG. 7B, the magnetic reluctance increases, with the result that the magnetic force acts in a direction in which the permanent magnets try to return to the position in which the magnetic reluctance is minimum. Therefore, a restoring force F acts on the permanent magnet (N-pole) 101. The restoring force F becomes larger as a shift amount from the position in which the magnetic reluctance is minimum as shown in FIG. 7A increases. As a result, when a stroke of the moving member lengthens, it is necessary that the elastic vibration member produces a thrust force for canceling the restoring force F in the position.
When an operation is performed in which the moving member which is stopping in a position shifted from the position in which the magnetic reluctance is a minimum as shown in FIG. 7A is returned toward the position in which the magnetic reluctance is a minimum by a very small amount, there is also a problem in that the moving member overruns an instructed position because of the restoring force F.
FIGS. 8A and 8B show an overrun of the moving member in a conventional linear vibration wave motor. FIG. 8A shows a drive pulse signal inputted to the vibration wave motor. FIG. 8B shows a change in moving speed of the moving member when the vibration wave motor is operated based on the drive pulse signal.
In FIG. 8B, a characteristic C0 indicates an ideal moving speed in the case where there is no restoring force F. On the other hand, a characteristic C1 indicates a moving speed in the case where there is the restoring force F. The characteristic C1 exhibits the overrun as compared with the characteristic C0.